bitcoin: implement is_scid_depth_announceable helper.

The math is a bit tricky, so encapsulate it.

Includes the extra 'e' in 'announcable' as noted by @cdecker :)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

bitcoin/short_channel_id.h

@@ -4,6 +4,7 @@
 #include <ccan/short_types/short_types.h>
 #include <ccan/structeq/structeq.h>
 #include <ccan/tal/tal.h>
+#include <common/gossip_constants.h>
 #include <stdbool.h>
 #include <stddef.h>
 
@@ -47,6 +48,16 @@ static inline u16 short_channel_id_outnum(const struct short_channel_id *scid)
 	return scid->u64 & 0xFFFF;
 }
 
+/* Subtly, at block N, depth is 1, hence the -1 here. eg. 103x1x0 is announceable
+ * when height is 108. */
+static inline bool
+is_scid_depth_announceable(const struct short_channel_id *scid,
+			  unsigned int height)
+{
+	return short_channel_id_blocknum(scid) + ANNOUNCE_MIN_DEPTH - 1
+		<= height;
+}
+
 /* Returns false if blocknum, txnum or outnum require too many bits */
 bool WARN_UNUSED_RESULT mk_short_channel_id(struct short_channel_id *scid,
 					    u64 blocknum, u64 txnum, u64 outnum);

lightningd/channel_control.c

@@ -376,9 +376,9 @@ void peer_start_channeld(struct channel *channel,
 
 	if (channel->scid) {
 		scid = *channel->scid;
-		/* Subtle: depth=1 at funding height. */
+		reached_announce_depth
-		reached_announce_depth = get_block_height(ld->topology) + 1 >=
+			= is_scid_depth_announceable(&scid,
-				       short_channel_id_blocknum(&scid) + ANNOUNCE_MIN_DEPTH;
+						     get_block_height(ld->topology));
 		log_debug(channel->log, "Already have funding locked in%s",
 			  reached_announce_depth
 			  ? " (and ready to announce)" : "");